Dielectric Studies of Organic Molecules and the Determination of Energy Barriers

Abstract

The relaxation times of polar molecules have been determined by the measurement of the dielectric absorption of dilute solutions of the polar solute molecules in a non-polar solvent at microwave frequencies, by a bridge method. The study was applied to molecules that contain rotating volar groups so that a group, as well as a molecular, relaxation time was evaluated. By measuring the relaxation times at several temperatures the enthalpies of activation to molecular and group rotation were determined. Among the compounds studied was acetophenone, which was examined in three solvents, and for which an energy barrier to acetyl group rotation of approximately 2 kcal/mole was obtained. Several other aromatic and also aliphatic molecules containing acetyl groups were investigated and the resulting relaxation time data correlated with the structures of the compounds. An intensive study was also carried out on aromatic molecules containing methoxy groups and the way in which the group relaxation time and energy barrier to rotation was affected by other substituents in the benzene ring noted. Dielectric studies have also been made on molecules of the type X-C,H,-X, where X is a rotating polar group, and for these only group rotation is observed. Since group relaxation is the only relaxation mechanism the relaxation time can be determined with an accuracy of + 5% whereas when molecular and group relaxation processes are both taking place the accuracy is approximately + 10% on the relaxation times. In addition to the derivation of the relaxation times of polar molecules the dipole moments have also been determined and these include several for which no previous value existed. The derived dipole moments have proved useful in assessing the distribution of charge within the molecules. The possible causes of the short relaxation times of diphenylether and its substituted derivatives are discussed. Selected molecules were chosen for study so that an estimation of the most likely causes could be made.